1. Field of the Invention
The present invention relates to downhole centralizers used in oilfield operations and, more particularly, relates to an improved centralizer for positioning on a drill string for use in a borehole with high angle build rates, such that the centralizer maintains the drill string centered within the build portion of a borehole yet allows high torque to be transmitted to the drill bit and produces a centralized means of stabilization and directional control.
2. Description of the Background
Both centralizers and stabilizers generally include a sleeve-shaped metal body for positioning downhole along a tubular string and an outer material sleeve for engaging sidewalls of the borehole. Centralizers have been used for decades in oilfield drilling operations to keep the tubular centered within a borehole or within a larger diameter tubular. Stabilizers are similar to centralizers, but are primarily used to maintain a desired path for the borehole or hold a drill bit on a desired course. As used herein, the term "centralizer" is intended to include both centralizers and stabilizers as those terms are commonly used in oilfield operations.
When drilling angle building wells, lengths of drill pipe have been interconnected with lobed mechanisms which allow angular movement of a lower joint of pipe with respect to an upper joint of pipe, thereby facilitating the bending of the drill string through the arcuate path of the build up. While drill strings comprising lengths of drill pipe interconnected by lobe-shaped mechanisms can be successfully used to transmit torque from the surface to a bit in a well with a high build-up rate, they do not result in the drill string being centered within the deviated portion of the borehole. Accordingly, high torque losses are encountered when the drill string rubs against the sidewalls of the borehole during the drilling operation, thereby causing excessive wear of the drill pipe.
Special problems are encountered using centralizers in wells with relatively sharp borehole deviations. Sharp or highly deviated boreholes are increasingly used, however, to recover hydrocarbons from subterranean formations at lower overall costs, or to recover more hydrocarbons in less time than conventional vertical boreholes. A centralizer ideally should keep the drill string centered with respect to the borehole through the curve of the deviation and/or maintain the drill bit on its desired course, but does not substantially increase the risk that the centralizer will become "stuck" in the borehole. Also, a centralizer should be able to transmit high torque loads and axially directed forces through the drill string to the bit, thereby enabling the drilling operation to be efficiently conducted. Also, a centralizer should be able to transmit high torque loads to the bit without substantial torque losses, so that less expensive torque generating equipment at the surface can be employed to obtain the desired torque on the bit, and so that the likelihood of failure of the drill string caused by high torque loads is minimized. Also, a centralizer preferably reduces the "walk" tendency of the drill bit due to reduced torque losses.
Drilling operators have heretofore often sacrificed the advantages of centralizers when drilling highly deviated boreholes, or have incurred increased cost to repeatedly trip in and out of the borehole to add or remove one or more centralizers from the drill string. In view of the increased use of deviated wells and wells which are vertically drilled to a desired depth and then highly angled for drilling horizontal or substantially horizontal boreholes, improved techniques are required so that operators can efficiently perform these drilling operations while obtaining the advantages of centralizers on the drill string.
The disadvantages of the prior art are overcome by the present invention, and an improved apparatus is hereinafter disclosed for use in a drill string which maintains the drill string centered within a deviated portion of a borehole and/or maintains the drill bit on its desired course while allowing high torque to be efficiently transmitted to the drill bit.